The epicutaneous test has been used for more than 70 years. The substance to be tested is applied on a patch and fixed to the skin with adhesive tape. The patch is left in place for 24 to 48 hours or more and then removed. If an eczematous response is elicited, the person probably has a contact allergy to the tested substance. Although always based on these principles, the testing procedures vary from one clinic to another. The conventional procedure of epicutaneous testing is very tedious and time consuming. Every patient has to be tested by the so-called "standard series" which is composed of about twenty allergens. Additional supplementary tests may be needed. The application of test substances on the patches is traditionally performed by hand. It is obvious that the procedure takes a very long time to achieve because the doses have to be dispensed as accurately as possible. Trying to be accurate in dosing, even when a trained staff is doing the work, is both tedious and tiresome. Accurate dosing is not possible by the conventional method however.
Patchtest technique has not as yet been subjected to systematic studies. There are no universally accepted procedures, test materials or norms for the concentration of test substances. Although the choice of suitable concentration of test substances is of fundamental importance, there are no universally accepted concentrations, and the concentrations differ from one clinic to another. The importance of standardized concentrations should be stressed upon since, for example, too high concentrations result in false positive reactions because of their primary irritant effect, and may even sensitize previously healthy patients, while too low concentrations produce false negative reactions.
Moreover, there are no norms for the vehicles in which the test substances are dissolved or norms for dosing the test substances which are applied on the patches.
In patch testing in test substances(allergens) applied are either
(1) as they are or PA1 (2) mixed with petrolatum or other ointment bases or PA1 (3) dissolved in water, olive oil, alcohol, acetone, methylethylketone (MEK), isobutylketone, butyl--or ethylacetate or liquid paraffin.
Different vehicles may give different results. With identical concentrations of perfumes in vaseline and in water, positive reactions were more frequent with vaseline. Some ointment bases cannot efficiently release certain substances. Solutions evaporate easily and the concentration of the test substance increases, resulting in false positive reactions and the concentration becomes uncertain. Volatile substances tend to diffuse from ointment bases resulting in false negative reactions. Hygroscopic substances become diluted by atmospheric humidity so that the concentration decreases and moreover they may degrade during storage. Some substances may change and degrade when exposed to light.
In order to elicit an allergic reaction in an organism already sensitized, not only is the concentration of the allergen important but also the amount of the allergen per unit area of the skin. The strongest reactions are obtained with 35 .mu.l of fluid. This is seldom taken in consideration which, from one clinic to another, vary considerably in size and hence also in the amount applied. The same thing i.e. not only the concentration, but also the amount per unit area can be of significance in inducing sensitization. It has been reported that the risk of patch-test sensitization increases with the amount of ointment or solution used. Moreover, large amounts of the allergen used may contaminate neighbouring test sites, giving rise to confusing test results.
Ointments for testing are kept either in glass jars or aluminium tubes.
The ointment dosages taken from the jars by a glass rod or expressed from aluminium tubes show striking degree of variation.
The amount of test solutions delivered by pipettes cannot be exactly measured and therefore show a great degree of variation. Application of too small doses of test substance must with certainity lead to false negative reactions; application of too large amounts may increase the risk of patch test sensitization.
Several materials, such as; linen, cotton and cellulose filter paper in different sizes, have been used. The size of the patch is not standardized. These test materials show up many disadvantages, for example the linen and cotton may react with the test substances, and the cellulose filter paper proved to reduce readily-oxidizable compounds, such as reduction of a hexvalent chrome(allergen) to the trivalent form(non-allergen) in a very short time.
Consequently, it may happen that a patient with chrome allergy does not react by a chrome patch test and is therefore declared free from chrome allergy.
Another disadvantage of the manual procedure of application of test substances from jars and tubes is that the order of tests should always be remembered. Not infrequently this order is being disturbed by the busy staff, giving rise to faults in the interpretation of test results and consequently to a wrong diagnosis. This serious fault may be suspected by the dermatologist when his clinical findings differ from the test results. This means that the tests have to be repeated. If the fault is not discovered then the problem is unsolved. The patient will avoid contact with wrong allergens and keep in contact with the actual ones. Medico-legal and compensation conflicts are to be expected in such cases.
The size of test area according to conventional testing methods varies considerably. It is about 1 cm or more in diameter. A large area of the skin is therefore needed to apply the "standard series" of about twenty test substances. Thus, a limited number of test substances can be tested. Additional tests cannot be performed without waiting for the recovery of the skin which has been previously tested.
When fixing the conventional patches onto the skin by adhesive tapes, it is usually difficult to keep them fixed because of sweating and body movement. Therefore, the necessary proper contact of test substances with the skin for 2 or more days is not alway successful. Tests are therefore often repeated.
The risk of exacerbation of the eczema during epicutaneous testing is common because the size of test area is too large. The test is also a tedious procedure.
Test patches of different kinds are already known through patent literature. They comprise a carrier with one or more places for the test substances. In the case where the carrier is provided with a test substance for later use, a covering film is applied on that carrier which hermatically encloses the substance. The covering film is removed immediately before the test patch is applied on the body.
A disadvantage with this type of carrier is that when the covering film is peeled off a larger or smaller portion of test substance follows with the protective film and this seriously affects the test result because it depends on an exact amount of test substance on an exactly defined surface. In order to solve this problem it even been suggested to apply the test substance on a tape and cover the substance with a piece of gauze or the like, whereby the test substance diffuses through the test patch when it is pressed against the body. This type of test patch requires special packing which protects the test substances together with the piece of gauze already applied on it. This makes the test device considerably more expensive especially when every single test substance must be packed separately.
Another drawback by the already used testpatches is that they do not accurately demarcate an exactly defined skin area, because the test substance tends to spread widely outside the carrier's test area. In the case where the carrier is gummed, even the adhesive layer would induce skin irritation, which would be difficult to differentiate it from the test reactions. One had tried to solve this problem by placing the test substances in tiny shallow covers which are fastened onto the skin by means of a tape. By this idea it was intended to press the cover's edge onto the skin and thereby limit the test area. But in practice it has been shown that the object could not be achieved without the use of a very strong adhesive, which normally gives rise to skin irritation. Moreover, these covers have shown up the same handicaps as mentioned before concerning the dosing, transport and storage of the testpatches.